"We have our share of that here," he said during a news briefing at his office in northwest Dallas.

Class, who took over in March, noted other American cities face similar threats.

He said federal agents scour Twitter, Facebook and other online channels looking for communications among Dallas-area residents and Islamic State-linked accounts.

"It's like searching for a needle in a haystack, but the haystack keeps getting bigger and bigger," said Ryan Young, assistant special agent in charge over intelligence.

Dozens of ISIS Twitter accounts are run by Web-savvy British fighters in Syria, officials say. They tweet to more than 18,000 English-speaking followers, many of whom are in America.

"Individuals in Syria are directing them to conduct their own attacks," using guns or knives if they don't have access to bombs, Young said.

Messaging in code

When users start messaging privately with Islamic State fighters, they exchange codes to talk on encrypted networks that prevent agents from intercepting messages.

"It's a technique ISIL uses very successfully right now," Class said, using another name for the extremist group, adding that it's hard for authorities to keep up with fast-changing technology. "We're always going to be a little bit behind."

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But the FBI said it's having success. In the six weeks before the Fourth of July, federal authorities arrested about a dozen people nationwide who are accused of conspiring with the Islamic State to carry out holiday attacks, Young said.

At least 56 U.S. residents have been arrested on charges related to Islamic State connections from March 2014 through late June, according to a report by the Center on National Security.

Three more suspects - including two gunmen who opened fire at a Muhammad art contest in Garland - were killed.

Of the 59 total suspects, 81 percent were U.S. citizens and 85 percent were males, according to the report. Their median age was 24.

On May 3, Elton Simpson, 31, and Nadir Soofi, 34, drove from Phoenix to open fire on the art event, wounding a Garland ISD security guard. A police officer and SWAT team returned fire, killing both gunmen. The Islamic State claimed responsibility for the attack.

Must be investigated

Later that month, the FBI arrested Bilal Abood, an Iraqi-born naturalized citizen who lived in Mesquite. The 37-year-old was accused of pledging allegiance to the Islamic State and trying to travel to Syria to fight with the group two years ago.

Alia Salem, executive director of the Dallas Council on American-Islamic Relations, said authorities are right to investigate any attempts by Islamic State to influence Americans.

"It's definitely a concern of ours that any child, whether it's a Muslim or a child of another faith tradition, comes in contact with people who mean to do them harm and do innocent people harm," she said. "We certainly look to the FBI to responsibly use legal means of investigating any types of threats to our community."